Active control of expiratory airflow by the respiratory muscles influences gas exchange, the degree of emptying of the lungs, and the work of breathing; may help optimize inspiratory muscle function; and is important during coughing and speech. Alterations in expiratory timing and velocity of flow occur in response to hypoxia, hypercapnia and exercise, and in diseases such as asthma, chronic obstructive pulmonary disease and sleep apnea. Recent studies of medullary and pontine respiratory centers suggest that neurophysiologically expiration may consist of tow distinct phases. In the proposed studies we will focus on the peripheral mechanisms by which abdominal, thoracic and upper airway muscles actively modulate expiration during these two phases. Specifically, in anesthetized dogs and cats we will 1) examine the manner in which multiple abdominal and thoracic expiratory muscles act in a coordinated fashion during resting and stimulated breathing; 2) study the relationship between the late expiratory activity of the above muscles and the early expiratory activity of the thoracic inspiratory muscles; 3) assess the extent to which upper airway dilators and constrictors are reciprocally active during the two phases of expiration, and how this os modified by respiratory stimuli; 4) record activity from single motor units of selected respiratory muscles, and examine how different motor unit subpopulations fire during expiration; and 5) assess the relationship between expiratory electrical activity of the above muscles and the mechanical consequences of muscle contraction, including respiratory changes in muscle length, expiratory flow rates and respiratory pressures. Studies will be performed during eupnea, when breathing is stimulated by hypercapnia and hypoxia, and following bronchoconstriction; in addition we will assess the roles that different chemoreceptors and mechanoreceptors play in mediating the responses to these stimuli. Finally, in chronically instrumented awake cats we will analyze expiratory activity recorded from several abdominal, thoracic and upper airway muscles to 1) ensure that conclusions derived from studies in anesthetized animals are not unduly influenced by the effects of anesthesia, and 2) assess the effects of spontaneous changes in posture on respiratory muscle activity during expiration.